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34:2 (2014)
Hen Harriers on Skye, 200012:
nest failures and predation
R.L. McMillan
Hen Harrier nesting success was studied on Skye from 2000 to 2012, during which period there
were 88 breeding attempts, 47 of which resulted in nest failures, with predation the most likely
cause. To obtain more accurate information on predation at Hen Harrier nests, four cameras were
used at nests between 2009 and 2012. Evidence from these cameras and post-mortem
examinations showed that predation by Red Foxes was the commonest cause of nest failure, 65%
of failures being attributed to foxes. In 75% of cases predation took place after dark. Foxes killed
two incubating adult females on the nest. Foxes mainly predated young from two to four weeks
old, but as recently fledged young return to the nest site to roost, they are also at risk. As fledglings
increased in size, nest sites became more detectable by foxes. In the absence of nest camera
evidence, it was often impossible to attribute a cause of failure, as no conclusive evidence was left
at the nest site. In one instance, foxes visited nest sites over a period of up to 10 days until all
the young were removed. At another site an adult fox brought cubs to a nest when small young
were removed. There was little evidence that adult Hen Harriers can successfully defend their
young against an incursion by a fox either in daylight or darkness.
Plate xx. Caption. Laurie Campbell
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34:2 (2014)
North
Skye
study area
Sleat
Strathaird
Struan
Portree
Waternish
South
Skye
study area
North
Skye
study area
South
Skye
study area
Figure 1. The Hen Harrier study areas on Skye, 200012.
Introduction
The Hen Harrier Circus cyaneus is a Scottish Biodiversity List Species and meets criterion 3a of
the Species Action Framework as a threatened species which is a focus of conflicts of interest with
stakeholders with other objectives notably game management. It is included in the statutory list
of the UK Biodiversity Action Plan. The Hen Harrier Conservation Framework (Fielding et al. 2011)
considered eight factors potentially affecting their distribution. Constraints ranged from grazing
pressure which might reduce the heathery habitat important for prey species, to wind farms. Illegal
persecution was a significant constraint in some areas, whilst in others, shortage of prey and
suitable nesting habitat were identified. Predation by mammals such as foxes, and avian predators
such as crows, was identified as a possible constraint on breeding success.
Hen Harriers have been studied on Skye, Highland, since 2000 where breeding sites have mainly
been associated with forestry plantations. Although persecution is a significant problem in some
parts of the UK, during the course of this Skye study, there was only one known instance of
human interference. This paper concentrates on nest failure rates and predation by Red Fox Vulpes
vulpes. Unlike other islands in the Hebrides, the fox is well distributed on Skye (Scott 2011).
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Study area
When the study commenced in 2000 all the known breeding sites were located in the Sleat and
Strathaird areas of south Skye, but the last of these was successful in 2004. Harriers were also
known to nest in north-central Skye and from 2005 a study area evolved, eventually extending
to 15,000 ha, lying north of the B885 Portree-Struan road and extending to Waternish. As in the
south, the North Skye study area was associated with forestry plantations and moorlands
immediately adjacent. Both study areas are shown in Figure1.
Methods
Fieldwork was conducted under licence issued by Scottish Natural Heritage (SNH) and followed
current best practice guidelines (Hardey et al. 2009), which recommend a minimum of four visits
to territories to check site occupancy, to locate nests early in incubation, to check for young, and
to check for fledged young.
Green & Etheridge (1999) acknowledged the high risk of predation to this ground nesting species
and emphasised the importance of observing and counting young in flight before determining
fledging success. Hardey et al. (2009), Whitfield & Fielding (2009) and Baines & Richardson (2013)
all provide examples of evidence found at nest sites which could allow causes of failure to be
determined. Attributing causes of failure can be extremely challenging, and not all types of
evidence found can be described as conclusive. A thorough search of nest surrounds was carried
out for evidential traces when a nest failure was discovered. However, with many nest failures
there was no conclusive evidence of causal factors.
In 2008, a successful grant application was made to SNH for nest site cameras in respect of an
action which improves, protects and manages native species and habitats. Four nest site cameras
were purchased from the RSPB Technical Department. Nigel Butcher of the RSPB provided
training and support; for further information on the technology see Bolton et al. (2007). Nest site
cameras were used at selected sites between 2009 and 2012.
Results
200508
Hen Harriers had been monitored on Skye for
nine years from 2000 to 2008. The mean failure
rate of nests was 51% (Table 1). Looking
exclusively at the North Skye data from 2005 to
2008, the failure rate was higher at 62%. The
percentage failure rate peaked at 80% in 2006,
when from the five pairs which laid eggs, 22
young hatched. Two broods failed at less than
two weeks and another two failed at greater
than two weeks, the latter suspected by fox
predation, and only two young fledged. Failure
rates in the next two years remained high and
there was growing concern that predation by
foxes was the main cause of failure (see Fig 2).
However, the evidence was weak, as it was
often impossible to attribute causes.
Plate 2. Evidence of Red Fox predation of Hen Harrier
chick, with bitten and chewed feathers in quill, Skye,
August 2008. R. McMillan
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Hardey et al. (2009) suggest that predation risk increases as young increase in size. If a predator has
taken large young, evidence may include bitten off feather quills and trails of down feathers. They
also conclude that there may be no signs to indicate small young are taken. In their study in Wales,
Whitfield & Fielding (2009) suggested that partially eaten remains or chewed feathers of the
incubating female and/or nestlings were assumed to be evidence of predation by a mammal (Figure
2). Whitfield & Fielding had also asked observers to record corroborative evidence of the predator
involved such as faeces, hair, feathers or distinctive scent. In the Langholm study, Dumfries &
Galloway (Baines & Richardson 2013), a number of criteria were applied to assign causes of failure.
In the Skye study, the only evidence found by the author was bitten and chewed feathers (Figure 1),
or dead young or adults. In the majority of failures, and despite intensive searches in the vicinity of
nests, there was no evidence of predation, which led to the decision to install digital nest-site cameras.
200912
The data shown in Table 2 from 2009 to 2012 were drawn exclusively from the North Skye study
area. Camera evidence was available for several nests each year, and this was supported by other
evidence such as chewed feathers and post mortem results. Over the four-year study, cameras were
installed at 61% of nests. During the four year period the overall nest failure rate was 57% and
based largely, but not exclusively, on camera evidence, 65% of failures could be attributed to foxes.
A total of eight camera activations resulted from fox intrusions, two when the young were less
than two weeks old, and six when the young were more than two weeks old. After 2009, cameras
were installed only on nests with young as this was considered to be the most vulnerable stage.
However, a female incubating eggs was killed by a fox in 2010, confirming that nests with eggs,
Table 1. Summary of Hen Harrier breeding success, all Skye, 200008.
Year Active Nests Mean Total Total Mean Failed Failed Total Cause
territories clutch young young fledged with with nests of failure
size hatched fledged per nest eggs young failed
2000 4 4 3.7 12 11 2.75 0 1 1 Predator
2001 3 3 3.7 5 5 1.66 1 0 1 Unknown
2002 5 3 2 2 2 0.66 1 1 2 Predator
Unknown
2003 8 7 ? ? 8 1.12 3 1 4 Predator
Unknown
2004 8 8 ? ? 20 2.5 2 0 2 Unknown
2005 10 9 5.4 21 16 1.8 1 2 3 Predator
Unknown
2006 8 5 5.4 22 2 0.40 0 4 4 Weather
Predator
2007 11 9 4.7 42 10 1.1 0 6 6 Predator
2008 11 10 4 28 8 0.8 1 6 7 Predator
Table 2. Summary of Hen Harrier breeding success, North Skye study area, 200912.
Year Active Nests Mean Total Total Mean Failed Failed Total Cause
territories clutch young young fledged with with nests of failure
size hatched fledged per nest eggs young failed
2009 7 6 5.6 16 9 1.5 2 2 4 Fox 2
Unknown 2
2010 12 10 5.4 36 10 1.0 1 6 7 Fox 6
Unknown 1
2011 12 9 5.25 25 15 1.6 0 3* 3 Fox 1
Unknown 2
2012 5 5 5.00 19 8 1.6 1 2 3 Fox 2
Unknown 1
*though RB3 was predated by a fox and two young killed, a single young fledged, so the site is regarded as successful.
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as well as brooding adults, can be vulnerable to foxes. In 2011, a nest was deemed to be
successful as a single young fledged despite its two siblings being killed by a fox on camera. Of
the eight recorded predations on camera, only four (50%), had other evidence to indicate that a
fox event had taken place. There was no evidence during the study of nests being visited by any
other mammalian or avian predators. In 2012, a single nest failed late in incubation and though
predation by a corvid could have been a factor, this could not be proved. Hooded Crows Corvus
cornix were frequent nesting neighbours. Figure 3 provides examples of the camera evidence.
Predation of incubating adults
In 2010, an empty nest was located on 16 May with an adult female dead beside it (Plates 4 & 5).
As this female was in moult, it was thought that it had been incubating a clutch of eggs and that
these were subsequently removed by the fox.
Plates 45. Dead female Hen Harrier at the nest, Skye, May 2010. R. McMillan Inset. Same dead female Hen
Harrier as Plate 4 (Skye, May 2010) at the time of the post-mortem, with a fox skull and canines superimposed on
the puncture marks. A.M. Wood
Plate 3. Intrusion of Hen Harrier nest by Red Fox, RBT site, Skye, 23 July 2010. (a) The initial intrusion at 02:24, then
(bc) fox returning at 21:28 to uplift the final dead youngster, which it had concealed. R. McMillan
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The carcass was examined by A.M. Wood, an avian specialist at the Animal Health and Veterinary
Laboratories Agency, Lasswade, Midlothian. The pattern of injuries found, with fairly symmetrical
distribution of puncture wounds on both sides just behind the wings/axillae, together with the
often paired puncture wounds in the skin and underlying muscles, suggested the bird had been
killed by a fox. In the case of this territory, the male was then successful in attracting another
female, which laid four eggs in a new nest some 200 m further west. Although four young
subsequently hatched, the nest was predated by a fox on camera (see Plate 3).
On 2 June 2011, a nest containing six small young was found and a camera was installed (RB4 in
Table 3). On 10 June, the nest was found to have been predated and the female was dead beside
the nest. All the small young had been removed. Again the carcass was examined by Mr. Wood.
The appearance was consistent with fox predation though maggot activity had enlarged some of
the wounds. The bites on the neck indicated an experienced animal rather than a cub.
In each case, the dead female was left beside the nest. Because the carcasses were discovered and
removed soon after the event, it is possible that a fox would have returned to uplift a carcass as food.
Discussion
Nest sites with cameras were visited more regularly than other sites. There is a view that foxes
may have been attracted to these nests by human scent and trampling of vegetation. The
introduction of larger batteries reduced the frequency of visits from every 5 or 6 days to every
7 or 8 days. As far as is practicable, walk-in approaches to sites were varied. Cameras were used
at 19 nest sites during the study and 52% of these nests were successful. Prior to the camera
study, between 2005 and 2008, of 31 nests which hatched young, only 14 nests were successful
(45%). There was therefore no evidence to suggest that the installation of cameras increased the
failure rate or predation risk.
Table 3. Evidence of Hen Harrier breeding success from nest site cameras, Skye, 200912.
Year Nest Date camera installed Result Remarks
code
2009 CH1 29 May on 6 eggs Failed on eggs Female deserted 6 June
(lack of provisioning)
RB1 29 May on 5 eggs Fledged 4 young
RB2 4 June on 3 yg & 2 eggs Fledged 5 young
GR1 4 June on 5 eggs Fox predation 4 June @ 23:15 All 5 young killed
2010 RBL 14 June on 5 young Fox predation 28 June @ 21:31 All 3 remaining young killed
RBU 14 June on 3 yg & 3eggs Fox predation 1 July @ 19:30 All 3 young killed, cubs brought to nest
FB 18 June on 6 young Fox predation (other information) Camera malfunction, but all young killed
CH1 18 June on 4 young Fledged 3 young
GR1 25 June on 4 young Fledged 4 young
RBT 12 July on 3 yg & 1 egg Fox predation 23 July @ 02:20 All 4 young killed
2011 RB4 2 June on 6 young Fox predation 4/5 June Incubating & & 6 young killed (infra-red
failure)
RB1 10 June on 4 young Fledged 4 young
CH1 10 June on 4 young Fledged 3 young
UIG 14 June on 4 young Fledged 3 young
RB3 29 June on 3 young Fox predation 13 July @ 02:00 Killed 2 young, but 1 on wing & fledged
2012 RB3 17.6 on 5 young Fledged 3 young Probably natural brood depletion
RB5 12.6 on 5 young Fox predation 2.7 @23:30 All 4 young killed
CH1 12.6 on 5 young Fledged 5 young
CH2 12.6 on 4 young Fox predation 22.6 @ 01:00 2 young killed & & defended
nest but fox returned 10 days later
& killed remaining 2 young
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In Galloway, Watson (1977) provided some evidence of fox predation. He also discussed nest
failures, but attributed this to crows rather than foxes and suggested that successful nesting had
occurred in areas where foxes are plentiful. In later years in Watsons study area, fox predation
became an increasingly important factor and may well have been implicated to some extent in the
extinction of forest-breeding Hen Harriers in Galloway by the mid-1990s (C.J. Rollie pers. comm.).
Simmons (2000) suggests that ground-nesting harrier species are susceptible to ground predators, with
between 9% and 52% of all nests in a sample drawn from seven harrier species, destroyed by predators.
He quotes a figure of 18% in a sample of 480 Hen Harrier nests from Picozzi (1984) in a study in
Orkney, where there are no foxes, although there are ground-nesting crows on the moorlands.
Green & Etheridge (1999) examined Hen Harrier breeding success in relation to grouse moors and
the Red Fox, but their data found no clear evidence for a beneficial effect of the control of foxes
and other predators by moorland gamekeepers, on Hen Harrier nest success. Redpath & Thirgood
(1997) provide some background to the long established study at Langholm. When fox and crow
control ceased on the grouse moor in 2000, it was suggested that higher numbers of generalist
predators impacted on other ground nesting birds including the hen harrier. (Baines et al. 2008).
A later paper (Baines & Richardson 2013) provided a breakdown of nest failure rates and causal
factors in the times when Langholm Moor was keepered and unkeepered, attributing predation
by foxes as the main cause of harrier breeding failure.
In Hen Harrier population studies in Wales, Whitfield & Fielding (2009) found that of 86 failed
breeding attempts where a cause of failure was attributed, 20 nest losses were due to fox.
ODonoghue (2012) found that predation was a significant issue in Ireland, with 19% of nests lost
to predators including foxes. Fielding et al. (2011) referred to previous work (Watson) which
suggested that variation in the aggressiveness with which harriers defend their nests may affect
the susceptibility of eggs and chicks to predation, as may the attentiveness of the parent birds,
which may be influenced by the availability of prey (Amar & Burthe 2001). In the Skye study,
there was no evidence that either of these factors makes any difference. The aggressiveness and
Plate xx. Caption. Laurie Campbell
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attentiveness of adult females may be a factor during daylight hours, but in the camera study, six
of the eight nest site intrusions by foxes occurred during the hours of darkness. In 2012, a female
harrier aggressively defended her nest to a fox intrusion during darkness but lost two chicks
initially, and the remaining two in the brood, two weeks later. Although not all harriers flee when
a fox intrusion occurs, evidence from this study suggests they offer little resistance to a persistent
fox attack and may well be at risk themselves. Though all the young may not be taken initially,
they will invariably be killed eventually. Of the two intrusions which occurred during daylight
hours, in one instance two live young were left in the nest until the fox returned with cubs two
hours later to remove them. The camera images failed to show any resistance by the adult birds.
In 2005, a webcam at a nest site at Clyde Muirshiel Park recorded the systematic reduction of a
brood of five young Hen Harriers to a single chick, which successfully repelled the fox (C.J. Rollie
pers. comm.). No chick on Skye has been recorded successfully repelling a fox.
The long-established practice of claiming birds have fledged at the point of ringing or when the
young are fully feathered may be unreliable, but remains part of guidance (Hardey et al. 2009).
In 50% of the camera activations in this study, there was no other evidence to suggest that there
had been a fox intrusion at the nest, in other words there had been a clean lift by foxes at the
nest. There is a risk that failures could be attributed to other causes. It might therefore be useful
to include nest failure rates as well as fledging rates, as indicators of the health of Hen Harrier
breeding populations. In areas not managed for Red Grouse, high failure rates at the chick stage
may well indicate a problem of fox predation.
Nest site cameras were deployed as part of the Langholm project between 2008 and 2013 on a
total of 11 nests, two of which failed, but with no evidence of fox intrusions. However, there had
been intensive control of foxes during this period (A. McCluskie pers. comm.). Whilst it might be
argued that foxes will be intensively controlled on all sporting estates, recent analysis of data
from Atholl Estates in Perthshire (McMillan 2011, pers. obs.) shows that predator control on some
grouse moors can be highly variable and directly relates to the efficiency/commitment of
individual gamekeepers. The possibility that Hen Harrier nests will be predated by foxes on some
grouse moors cannot be excluded, and in the context of Langholm, it appears that in 1999, when
grouse moor management was still in place, there was a sudden occurrence of four nest failures
attributed to foxes, in addition to which, two adults were killed on nests (Baines & Richardson
2013). There had been no fox events in the preceding six years of the study. In the subsequent
unkeepered phase of the study 200007 there was an average of a single identified fox event
each year. At Langholm, 19932007, there were a total of 130 breeding attempts from which there
were 41 nest failures and 56% of these occurred when the birds were on eggs. In the Skye study,
200012, there were 88 breeding attempts with a total of 47 failures, 28% of which were on eggs.
The contrast between the two datasets is interesting. The general view amongst raptor workers is
that most fox intrusions occur when broods are at the later stage of development and that is
supported by results from the Skye study. This is not to say that fox predation did not occur at
Langholm, but human disturbance, deliberate or otherwise, may have been a factor given the
disproportionate number of failures on eggs.
Whilst Fielding et al. (2011) acknowledge that foxes can be important predators they were not
included as a predictor in the harrier distribution model. They also concluded that fox impact seems
to be largely on harrier productivity rather than distribution, supported by observations by P.
Haworth in Kintyre, where breeding harriers and foxes appeared to be abundant. However, these
observations are at odds with studies of two areas of Mid-Argyll and north Kintyre, where breeding
birds have disappeared and this has been attributed to a large proportion of failures due to predation,
possibly by foxes (ap Rheinallt et al. 2007). J. Halliday (pers. comm. 2013) has updated these data
showing that four territories in Kintyre occupied in 19972008, were abandoned and this he
attributed to a high incidence of failure due to predation, with foxes the most likely predator. Also
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in Argyll at the Moine Mhor NNR, J. Halliday reported that up to three pairs had bred between 1989
and 1996 when the territories were abandoned. Although birds returned in 2010 and 2011, there were
further failures and J. Halliday suspected that high rates of predation, probably by foxes, and lack
of recruitment to the population also explained the demise of this population. In a study in Cowal,
Argyll, from a population of eight pairs in 2003, only a single pair bred successfully in 2013 and
fox predation is considered to be a factor in the population decline (A. French pers. comm.). During
the course of this study, breeding pairs disappeared from south Skye with the exception of 2011,
when a pair returned after a 10-year gap, but failed.
Madders (2010) noted that the extent of first rotation forestry was in decline in the west of
Scotland and predicted that the loss of this habitat would lead to a reduction in numbers of
harriers breeding. New et al. (2011) modelled the population dynamics at Langholm. In afforested
areas, as the canopy closes and small mammals reduce in numbers, New et al. suggest that in the
long term the population size could not be maintained as there would not be enough young
harriers to replace the older birds lost to the population. However, in the context of the Skye study,
whilst there have been some population fluctuations, clutch and initial brood size has remained
high, suggesting that lack of food has not been a factor. The difficulty has been brood survival
and nest failures as a result of fox predation. In the context of the New model, with fewer young
birds fledging, or occasional breeding adults predated by foxes, the problem of recruitment to the
breeding population would be exacerbated. Evidence from Skye, and a number of other areas,
suggests that successive failures as a result of predation may contribute to the abandonment of
territories, making populations less viable, and in some regions directly impacting on distribution.
Digital nest site cameras have proved to be an extremely useful research tool and the images,
along with other material found at nest sites, provides compelling evidence that fox predation of
Hen Harrier nests on Skye is considerably higher than that apparently recorded in any other area
of the UK or Ireland. The technology would seem critical for any future research into establishing
causes of nest failure or brood depletion.
Hayhow et al. (2013) reported that the breeding population of Hen Harriers in the UK and Isle of
Man declined between 2004 and 2010. They concluded that a 20% decrease in Scotland may be
related to habitat change and illegal persecution. It appears that further declines occurred in 2012
which has prompted SNH to commission additional work on the Hen Harrier Conservation
Framework (SNH 2013). In the Skye study, only three nests were found in 2013, the lowest number
recorded since the study commenced.
Problems of human persecution remain a priority for the conservation of this species. However, it
cannot be the exclusive focus when there are clearly other problems in forests and moorlands not
managed for other sporting interests. The challenges facing Hen Harriers are complex, but when
high nest failure rates occur there is a need for a detailed interpretation of causal factors.
Acknowledgements
I am grateful to Ken Crane and Kate Nellist for detailed observations in the study area. Scottish
Woodlands, Forestry Commission Scotland and J. Robin Dixon provided unrestricted access to the
study area. Access to windfarm roads at Ben Aketil and Edinbane was extremely helpful. The
ongoing monitoring study has been part-funded by Haworth Conservation Ltd. and Ben Aketil
Wind Energy Co. The nest site camera study has been grant-aided by Scottish Natural Heritage,
supported by Atmos Consulting and Haworth Conservation Ltd. Mr A.M. Wood of VLA Lasswade
carried out post mortem examinations of dead adult birds and young. The images are by the
author and Mr. A.M. Wood. I am grateful to other harrier enthusiasts for many useful discussions.
Nigel Butcher of RSPB has provided ongoing help and technical support. Paul Haworth and an
anonymous referee have provided helpful comments on early drafts of this paper.
39 Scottish Birds
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R.L. McMillan, Askival, 11 Elgol, Broadford, Isle of Skye IV49 9BL.
Email: bob@skye-birds.com
Revised ms accepted February 2014